Semiconductor wafers are shipped to a processing site in a vacuum sealed container. The wafers are then unpacked and processed in clean rooms. Care must be taken since particulate matter on a wafer will flaw or contaminate the wafer and subsequent integrated circuit chips built on the wafer, causing circuit defects and reducing reliability.
Federal and industrial standards have been established for the allowable concentration of particulate matter in the atmosphere of various clean rooms. Both for the purpose of remaining within a particular standard and for the purpose of self-regulating quality control, it is necessary to regularly test the clean room to determine the concentration of particles in the atmosphere.
Devices for collecting particulate matter from an atmosphere are known. U.S. Pat. No. 3,711,707 to Lilienfeld et al. is typical of the prior art collectors. The patent describes a monitoring apparatus which collects aerosol particles by impaction. Particle impactors operate under the principle that as an air stream is caused to strike a substrate, the heavy particles impact upon the surface because of their mass.
U.S. Pat. No. 3,970,428 to Barringer operates in much the same manner as particle impactors. The substrate, however, is replaced by a strip of tape having an adhesive coating to capture the particles. The strip of tape is advanced periodically so that the particulate matter is collected in a number of "dots" or circular areas. U.S. Pat. No. 3,458,974 to Orr, Jr., et al. describes a collector which captures particles by impinging a heated gas on a cooled surface. The temperature gradient causes the particulate matter to be driven toward the cooler surface by thermal forces, whereupon the particles are collected.
Conventional prior art collectors and detectors operate only where relatively high concentrations of particles or relatively heavy particles are encountered. Additionally, some conventional devices require the substrate to be mechanically fastened to the devices. Mechanical fastening may cause particulate matter to enter the atmosphere.
Some impactors center the collected particles in one small area, thereby making it more difficult to determine the size of individual particles. Barringer and Orr, Jr., et al. solve this problem by moving the tape or substrate, but the particulate matter is still collected only where the air stream is directed.
It is an object of the present invention to provide an apparatus for collecting particulate matter from an ambient atmosphere that is capable of collecting matter smaller than that collected by conventional impactors and is able to provide a suitable measurement surface. A further object is to standardize the gas flow rate in such an apparatus despite fluctuations of pressure from a vacuum source, thereby making data from testing as a function of time more reliable.